kernel - x86-64 - normalize the location of the pcb/md_regs and remove gd_rsp0
* Normalize the location of the pcb and related user process register
frame for traps and system calls.
* The pcb is now 16-byte aligned at the top of the kernel thread's stack
and md_regs is placed just below it.
* We no longer reserve 16 bytes for a 8086 emulation frame.
* Change the md_regs assignment in the trap code to an assertion that the
value is correct.
* Remove gd_rsp0, use the value from common_tss.tss_rsp0 instead (which
is the same).